1. Field of Invention.
This invention relates to surface acoustic wave (SAW) devices and more particularly to SAW devices which are provided with unidirectional input transducers.
2. Prior Art.
SAW devices are well known in the art as being particularly adapted to signal filtering applications. One of their main advantages is that they can be constructed utilizing the identical photolithographic techniques which are employed to produce monolithic integrated circuits and when so constructed, are compatible with such circuits.
The design of a SAW device is essentially a tradeoff wherein one design parameter is balanced against another to achieve the desired result. For instance, a SAW device's insertion loss can be reduced by adding an input tuning circuit which balances and cancels the reactance of the SAW device. This has most often been suggested with SAW devices which employ bidirectional input transducers. Such circuits are shown in U.S. Pat. Nos. 3,582,837; 4,007,433 and 3,827,002. While the suggested tuning circuits provide the desired result, i.e., the elimination of the capacitive reactance inherent in the input transducer, they have no effect upon the triple transit loss which is also inherent when bidirectional transducers are employed. Triple transit losses arise as a result of the subtraction between a reflected surface wave in the SAW device and the applied input signal-resulting in a lessened effective input signal.
A well-known solution to the triple result loss problem is the substitution of a unidirectional input transducer in place of a bidirectional input transducer. A unidirectional transducer has the property of causing a wave to propagate only in one direction upon energization of its inputs with quadrature related input voltages. It has the further property of being substantially insensitive to reflections from the output transducer and thereby largely eliminates the problem of triple transit signal reflections. Unidirectional transducers are well-known in the art and examples may be found in U.S. Pat. Nos. 3,800,248; 3,845,419 and 3,866,154.
In order to achieve the requisite phase relationship at the inputs of a unidirectional transducer, the prior art simplistically suggests the imposition of a 90 degree phase shifter between the source of signal and one of the transducer's inputs. 90 degree phase shifters are known in the prior art but as a general rule, require a number of components to achieve the required phase shift; add to the insertion loss of the SAW device; and furthermore, create an imbalance in the voltage applied to the device's input ports.